The magnetic field and pressure dependence of the magnetic ordering transition in NaxCoO2 (0.6<x<0.72)

TL;DR

This study investigates how magnetic ordering temperature in NaxCoO2 varies with magnetic field and pressure across different sodium concentrations, revealing unexpected pressure effects on magnetic order.

Contribution

It provides the first detailed analysis of pressure dependence of Tmag in NaxCoO2, showing an increase under pressure contrary to typical spin density wave models.

Findings

Tmag remains constant with decreasing x until x=0.65

Tmag is unaffected by magnetic field at x=0.70

Tmag increases with applied pressure from 22.0 K to 25.4 K at 10 kbar

Abstract

We have measured the magnetic field (H<90 kOe) and pressure (P<10 kbar) dependence of the magnetic ordering temperature, Tmag, in single crystal samples of NaxCoO2 for a range of Na concentrations (0.60<x<0.72). We show that in zero field, Tmag remains constant with decreasing x before magnetic order disappears at x=0.65. Heat capacity and magnetization data show that for x=0.70, Tmag is unchanged in an applied field. In contrast, magnetization data collected under hydrostatic pressure show that Tmag increases from 22.0 K at 1 bar to 25.4 K at 10 kbar. This rise is at odds with the behaviour expected for model spin density wave systems.